Spinal bone plates are used for a variety of conditions to immobilize, stabilize or align cervical vertebrae. For example, after a spinal fusion surgery, bone plates are used to add strength and rigidity to the adjoined vertebrae. Also, plates secure vertebrae together where an intervening vertebra has been removed or replaced. In other cases, spinal bone plates are used to correct instability in the spine caused by trauma, tumors, advanced degenerative discs, infection or congenital or acquired deformities.
A typical spinal bone plate includes an elongated rectangular plate that spans the distance between two or more vertebrae. The plate is curved to match the natural curvature of the spine at the location to which it is attached and bone screws are used to fasten the plate to the vertebral bodies. A pair of apertures is formed at one end of the plate for passing bone screws through and into a first vertebral body to secure the first end of the plate to the first vertebral body. A second pair of apertures is formed at the other end of the plate for passing bone screws through and into a second vertebral body to secure the second end of the plate to the second vertebral body. Thereby, the plate bridges two vertebral bodies. More vertebrae may be connected with a longer plate and a corresponding increased number of bone screw apertures and bone screws inserted therethrough at the intervening vertebral levels.
Spinal stabilization techniques can employ bone plates on the posterior, anterior, lateral, postero-lateral, and antero-lateral portions of a spinal column to provide fixation of the spinal column for the repair of injured or diseased vertebrae, intervertebral discs and other elements of the spinal column. Holes are drilled into the vertebral bodies or self-tapping screws are employed. The plate is properly aligned on the vertebrae. Proper alignment of the plate includes selecting the correct spacing between the upper bone screws and the lower bone screws. If fixed-length plates are employed, alignment includes selecting the plate with the correct length. Variable-length plates may also be employed in which an upper portion of the plate moves longitudinally relative to a lower portion for custom length adjustment. Mounting screws are inserted through the plate and the plate is carefully and firmly attached to the bone. Sometimes fusion is accompanied by a discectomy in which a herniated disc is removed and a graft device is placed between the vertebral bodies to assist in fusion across levels. With the plate in position, the vertebrae are held by the plate in desired spatial relationships and orientations relative to each other, pressure is removed from the nerve roots and pain caused by the herniated disc or other condition is relieved.
Over time, the interface between the screws and the bone may present some problems of stability. Due to the anatomical structure of the spine and the extreme anatomical forces that are brought to bear on the skeleton and transmitted to the spine, the screws securing the plate to the spine may vibrate or toggle out of position. Also, the degeneration of vertebral bone quality may result in the screws loosening or becoming dislodged. As a result, bone screws securing the plate to the spine may move or back out of the vertebral body and plate.
Therefore, there is a need to provide a new and improved bone plate that resists fasteners, such as bone screws, from backing out of the plate and also from being loosened with respect to the plate before migrating out. Not only an improved and effective fastener retaining mechanism is required, but also, an improved expandable plate that allows for small-increment variability in its length. A properly aligned plate as a result of custom length adjustment will improve force distribution and reduce fastener migration. Furthermore, there is a need for the spinal plate to withstand anatomical forces and be easily implanted. The screw-retaining mechanism must be easily activated by the surgeon, and also, the variable length adjustment must be easily accomplished without multiple steps. This invention, as described in the detailed description, sets forth an improved spinal plate system with anti-back out protection and variable length adjustment that meets these needs.